The traditional heat pump theoretical basis is originated from the discovery by Carnot in the early 19th century, and he published an essay about Carnot cycle in 1824. In 1852, Thomson put forth the first time his envisage of heat pump with air as working medium; and in 1854, Kelvin stated that a refrigerating apparatus could be used for heating. In the 1870s, refrigerating equipment based on Carnot principle developed rapidly, however, the development of heat pump lagged far behind. The world energy crisis in 1973 made all countries in Europe as well as the Soviet Union, Japan, the United States and Australia attach great importance to the work on heat pump. Today, all countries in the world are becoming increasingly interested in heat pump, and the manufacturers in Europe, Japan and North America have supplied many heat pumps for industries, commerce, building and civilian applications. International organizations, such as the International Energy Agency and European Community, have worked out development programs for large heat pumps, the testing of many new technologies and the popularization and application of existing heat pump technologies in new fields are going on or under planning, and the applications of heat pump are daily widening. Heat pumps are playing a daily increasing important role in energy conservation.
A traditional heat pump takes up heat from the surrounding environment, and transfers it to the object to be heated up, it follows the same working principle as a refrigerator as the reverse cycle of a heat machine, with the only difference in working temperature range. In some industrial sectors requiring heat supply and refrigerating at the same time, such as meat processing, food and dairy processing, the use of heat pump apparatuses for comprehensive cold and heat supplies is more economic and rational.
In a heat pump type household air conditioner used in winter, the massive cold obtained in the circulation of the refrigerator is only used to absorb the waste heat from the surrounding environment by the outdoor evaporator, and this is actually a great waste; for heating in winter with low ambient temperature, high humidity and in damp and rainy weather, the heating efficiency decreases seriously with ordinary air source heat pump air conditioners available on market, and they sometimes even cannot start up and work normally; therefore, in the north of China where it is quite cold in winter, ordinary air source heat pump air conditioners can only be used in transitional seasons, and they basically cannot meet the heating demand in cold winter. It is known to all that in the north of China, the traditional centralized heating is mainly by burning coal and gas, and such heating form cannot meet the requirements of social development in energy conservation, environmental protection or safety. Therefore, it is necessary to develop heat pump air conditioners that can function normally in winter with severe cold, and the corresponding auxiliary equipment and systems should also be provided.
The traditional refrigerating theory is mainly based on thermodynamics, i.e. Carnot reverse cycle of identical temperature difference is used to analyze the refrigerating cycle process, the economic indicator of the refrigerating cycle is the refrigeration coefficient, or the ratio of obtained gain to the cost of consumption, and also, of all refrigerating cycles between atmospheric environment with temperature for T0 and low temperature heat source with temperature of Tc (such as refrigeration store), the reverse Carnot cycle has the highest refrigeration coefficient:
                              ɛ          c                =                                            (              COP              )                                      R              ,              C                                =                                                    q                2                                            w                0                                      =                                          T                c                                                              T                  0                                -                                  T                  c                                                                                        (        1        )            
In the formula above, εc is the refrigeration coefficient, q2 refrigerating capacity of the cycle, and w0 the net work consumed by the cycle.
In fact, in his thesis “Reflections on the Motive Power of Heat”, Carnot concluded that: of all heat engines working between two constant temperature heat sources of different temperatures, the reversible heat engine has the highest efficiency.” This was later referred to as the Carnot theorem, after rearranging with the ideal gas state equation, the thermal efficiency of Carnot cycle obtained is:
                              η                      c            ⁢                                                                =                  1          -                                    T              2                                      T              1                                                          (        2        )            
In Formula (2), temperature T1 of the high temperature heat source and temperature T2 of low temperature heat source are both higher than the atmosphere ambient temperature T0, and the following important conclusions can be obtained:
1) The thermal efficiency of Carnot cycle only depends on the temperature of high temperature heat source and low temperature heat source, or the temperature at which the media absorbs heat and release heat, therefore the thermal efficiency can be increased by increasing T1 and decreasing T2.
2) The thermal efficiency of Carnot cycle can only be less than 1, and can never be equal to 1, because it is not possible to realize T1=∞ or T2=0. This means that a cyclic engine, even under an ideal condition, cannot convert all thermal energy into mechanical energy, of course, it is even less possible that the thermal efficiency is greater than 1.
3) When T1=T2, the thermal efficiency of the cycle is equal to 0, it indicates that in a system of balanced temperature, it is not possible to convert heat energy into mechanical energy, heat energy can produce power only with a certain temperature difference as a thermodynamic condition, therefore it has verified that it is not possible to build a machine to make continuous power with a single heat source, or the perpetual motion machine of the second kind does not exist.
4) Carnot cycle and its thermal efficiency formula are of important significance in the development of thermodynamics. First, it laid the theoretical foundation for the second law of thermodynamics; secondly, the research of Carnot cycle made clear the direction to raise the efficiency of various heat power engines, i.e. increasing the heat absorbing temperature of media and lowering the heat release temperature of media as much as possible, so that the heat is release at the lowest temperature that can be naturally obtained, or at the atmospheric temperature. The method to increase the gas heat absorbing temperature by adiabatic compression is still a general practice in heat engines with gas as media today.
5) The limit point of Carnot cycle is atmospheric ambient temperature, and for refrigerating process cycles below ambient temperature, Carnot cycle has provided no definite answer.
Because of the incompleteness of refrigeration coefficient, many scholars at home and abroad conducted research on it, and proposed methods to further improve it. In “Research on Energy Efficiency Standard of Refrigerating and Heat Pump Products and Analysis of Consummating Degree of Cyclic Thermodynamics”, Ma Yitai et al, in conjunction with the analysis of introduction of the irreversible process of heat transfer with temperature difference into heat cycle by Curzon and Ahlborn and the enlightenment from the finite time thermodynamics created on it, as well as the CA cycle efficiency, proposed the consummating degree of thermodynamics, advancing to a certain extent the energy efficiency research on the refrigerating and heat pump products.
However, the basic theory of thermodynamics cannot make simple, clear and intuitional explanation of the refrigerating cycle. Einstein commented the classical thermodynamics this way: “A theory will give deeper impression to the people with simpler prerequisite, more involvement and wider scope of application.” In the theoretical interpretation in the refrigeration field, this point should be inherited and carried forward.
Therefore, it has become a difficult point in the research in air conditioning refrigerating field to really find a correct theoretical foundation of refrigerating cycle, propose a new cold regenerative air conditioning apparatus on this theoretical foundation and apply it in practice, to effectively increase the conversion efficiency of air conditioning apparatuses in refrigerating in summer and heating in winter.